The superparamagnetic effect poses a serious challenge for continuing to increase the areal density and storage capacity of disk drives. One of the most promising methods to circumvent the density limitations imposed by this is the use of patterned media. In conventional media, the magnetic recording layers is a thin film of magnetic alloy, which naturally forms a random mosaic of nanometer scale grains which behave as independent magnetic elements. Each recorded bit is made up of many of these random grains. In pattern and media, the magnetic layer is created as an ordered array of highly uniform islands, each island capable of storing an individual bit.
In conventional media, bit cells are fabricated on circular tracks on a disk. Each bit cell comprises many tiny magnetic grains. Each grain behaves like an independent magnet whose magnetization can be flipped by a write head during the data writing process. These grains are irregularly shaped and randomly oriented. If the grains are small relative to the size of the bit cell, the magnetic transitions are straight enough so that it is easy to detect the boundary between adjacent bit cells. Shrinking the bit cells to increase areal density, however, without shrinking the grain size makes the magnetic transitions harder to detect.
The traditional solution to this problem has been to shrink the grain size. However, there is a practical limit. The magnetization of very small grains is unstable. According to the superparamagnetic effect, the magnetization of a grain can flip spontaneously if the product of the grain volume and its anisotropy energy falls below a certain value. The result is a loss of data.
In patterned media, each bit is stored in a single deliberately formed magnetic switching volume. This may be one grain, or several exchange volume coupled grains, rather than a collection of random decoupled grains. Single switching volumes magnetic islands are formed along circular tracks with regular spacing. Magnetic transitions no longer meander between random grains but are distinct boundaries between precisely located islands.